first, try keep him at range. second, as soon as he throes a grenade hack at you, shoot it before it explodes. third, don’t linger around after he dies, because he’ll have a few activated grenades on him, just in case you cant help loot him immediately.
Zombie grenadiers almost always drop some form of live explosive manhack on death. If you shoot one down, run the other way before they go off. If you melee one down, run the other way before they go off. In short, run the other way whenever you take one down to avoid getting blown up. They’ll leave a few undetonated ones behind to pickup afterwards though.
Kill it fast with a ranged weapon, then get some distance (in a lab, try closing one of those strong metal doors between you and it) and wait for the boom
Kill it fast with melee, grab the (active) explosives, and throw them away from you ASAP. This is still fairly dangerous!
Take your time, kill each explosive whatsit-hack as they come out (preferably with a ranged weapon). Eventually, the grenadier will change from “Tracking” to “Hostile”. When this happens, it means he’s out of explosives. Now you can kill it without the boom afterwards (even though it may still drop some inactive explosives…).
Get something else to kill it, come back later. After the first death/boom, no more explosives, even after it revives. Good old nailboard traps work pretty well in a lab.
For ELITE grenadiers, number 3 takes FOREVER, since they have so many hacks, but it is really the only safe way to do it (well, 4 can also work), since they have one mini-nuke hack (which can be one of the (active) ones on death if they haven’t used it yet!!).
What I did when in the lab was, since i had opened the armory, I lured the zombie to the door and closed it before it could follow, trapping it safely inside as I waited for the grenade hack it had spawned to blow up, killing it. When you don’t have an impenetrable steel door available, I’d just shoot it, run, or wait for it to blow itself up.